The main objective of the Radiological Physics Center (RPC) is to assure the NCI that the radiation dose delivered to patients entered onto interinstitutional clinical trials is clinically comparable. This is accomplished ultimately by a review of the radiation treatment records of individual patients entered onto these clinical trials. The RPC verifies that the treatment reported was in fact delivered and, in conjunction with cooperative group clinicians, verifies that the treatment was within the protocol specifications. The dosimetry data used for these reviews is obtained by an on-site dosimetry review of the participating institutions by one of the RPC physicists. Key radiation therapy staff are interviewed, typical treatment cases are assigned and evaluated, and dosimetry measurements are made on the therapy units. Mailed thermoluminescence dosimeters (TLD) are evaluated on a periodic basis to identify potential calibration discrepancies and to assure the RPC that the institution has not modified its dosimetry practices since the RPC on-site visit or the last TLD evaluation. The RPC continuously modifies its techniques to reflect common practice at participating institutions. Conventional megavoltage x-ray, megavoltage electron and low dose rate brachytherapy dosimetry are routinely monitored. Special procedures monitored include high dose rate brachytherapy, total body photon therapy, stereotactic radiosurgery, and conformal therapy. Whenever discrepancies are discovered, the RPC works with the institution's staff to bind an acceptable solution to resolve the discrepancy. The RPC provides these services for all 9 cooperative groups, and collaborates with 4 conventional radiotherapy quality assurance offices and 2 specialized quality assurance offices (3-D and protons) that serve these cooperative groups. The RPC also provides advice to the cooperative groups on quality assurance questions, including assistance in drafting the radiotherapy portion of treatment protocols. Research is performed on problems associated with multiple institutions using multiple energies and modalities to deliver clinically comparable doses. There are currently 1063 megavoltage therapy facilities monitored by the RPC. The staff of the RPC includes 8.1 FTE professional staff and 15.0 other staff. Approximately 27 percent of the personnel budget relates to on-site dosimetry review, 21 percent to the TLD service, and 21 percent for chart review. The remainder of the personnel budget relates to formal interactions with the cooperative groups and other activities such as computer programming and statistical support.